Pressurized vessels have been used for many years. One form of pressurized vessel has a gas-tight liner with a cylindrical central section and two rounded, generally semispherical end portions. It is common for a port to be located at one longitudinal end of the vessel, at the center of one of the semispherical ends. Fibers are wound around the exterior surface of the liner, thereby strengthening the liner to form a vessel capable of containing pressurized contents. Such pressurized contents may be water, or other liquids, or gasses.
In construction, the liner is generally molded from plastic. One common form of molding liners is blow molding, where plastic (or other desirable liner material) is extruded or pre-formed into a tube or other desirable shape and then placed in a mold at an elevated temperature. Air is then forced into the center of the plastic, thereby forcing the soft plastic outward against the walls of the mold. The outer surface of the plastic takes the shape of the mold, while the inner surface of the plastic loosely follows the contours of the outer surface, thus forming a generally smooth inner surface.
One common problem with blow molded containers is the development of thin spots due to stretching of material around corners of the mold. The thin spots create weak portions of the liner, thus limiting its capability to hold pressurized material.
One feature that is used during the manufacturing process of the pressure vessels described above is a “foot” located at one longitudinal end of the liner (usually the end opposing the port of the container). The liner is gripped at the foot during winding of the fibers. Some manufacturers mold the liner and then affix a foot to the liner after molding. The extra step of adding the foot adds cost and time to the liner making process. To overcome this, some manufacturers have tried to mold a foot onto the liner during the blow molding process. Thin spots are located proximate the inside corners and the center of the foot. These thin spots lead to weakening of the liner, and thus decreased stress resistance and reduced performance of the pressure vessel.
Generally, resin coated fibers are wound around the liner in various directions. Fibers are wound radially around the tubular center section of the liner. The radial windings may form one or more layers and usually do not cover the end portions of the liner. Fibers are also wound longitudinally around the liner. The longitudinal windings are generally wrapped around the ends of the vessel in a number of directions, thereby encircling the foot and providing structural reinforcement to the covered portions. Radial windings may be applied first, followed by longitudinal windings, or vice versa. The windings are wrapped tangentially around the foot to cover the entire end of the vessel. Once the fibers are wrapped, the resin is cured to provide a strong pressure vessel.
Generally, fibers are not wound over the foot, leaving an unreinforced portion at one end of the liner. Thus, the foot presents a potential weak spot in the pressure vessel. It would be beneficial to provide a vessel design which minimizes the unreinforced area and thin spots in the liner.